﻿function togbk(strUtf8)
{
        var bstr = "";
        var nTotalChars = strUtf8.length;        // total chars to be processed.
        var nOffset = 0;                                        // processing point on strUtf8
        var nRemainingBytes = nTotalChars;        // how many bytes left to be converted
        var nOutputPosition = 0;
        var iCode, iCode1, iCode2;                        // the value of the unicode.

        while (nOffset < nTotalChars)
        {
                iCode = strUtf8.charCodeAt(nOffset);
                if ((iCode & 0x80) == 0)                        // 1 byte.
                {
                        if ( nRemainingBytes < 1 )                // not enough data
                                break;

                        bstr += String.fromCharCode(iCode & 0x7F);
                        nOffset ++;
                        nRemainingBytes -= 1;
                }
                else if ((iCode & 0xE0) == 0xC0)        // 2 bytes
                {
                        iCode1 =  strUtf8.charCodeAt(nOffset + 1);
                        if ( nRemainingBytes < 2 ||                        // not enough data
                                 (iCode1 & 0xC0) != 0x80 )                // invalid pattern
                        {
                                break;
                        }

                        bstr += String.fromCharCode(((iCode & 0x3F) << 6) | (         iCode1 & 0x3F));
                        nOffset += 2;
                        nRemainingBytes -= 2;
                }
                else if ((iCode & 0xF0) == 0xE0)        // 3 bytes
                {
                        iCode1 =  strUtf8.charCodeAt(nOffset + 1);
                        iCode2 =  strUtf8.charCodeAt(nOffset + 2);
                        if ( nRemainingBytes < 3 ||                        // not enough data
                                 (iCode1 & 0xC0) != 0x80 ||                // invalid pattern
                                 (iCode2 & 0xC0) != 0x80 )
                        {
                                break;
                        }

                        bstr += String.fromCharCode(((iCode & 0x0F) << 12) |
                                        ((iCode1 & 0x3F) <<  6) |
                                        (iCode2 & 0x3F));
                        nOffset += 3;
                        nRemainingBytes -= 3;
                }
                else                                                                // 4 or more bytes -- unsupported
                        break;
        }

        if (nRemainingBytes != 0)
        {
                // bad UTF8 string.
                return "";
        }

        return bstr;
}